1. Field of the Invention
The present invention relates to hopped malt beverages, especially alcoholic brewery beverages produced at least in part from malt, and to improvements in the flavor stability thereof. More particularly, the present invention relates to imparting to hopped malt beverages improved stability against the production of thiols associated with a "skunky" odor and flavor development in beer that has been exposed to visible light.
2. Description of Related Art
As is well known and accepted in the malt beverage brewing art, subjecting a hopped malt brewery beverage, especially an alcoholic hopped malt brewery beverage such as lager, ale, porter, stout, and the like (herein generically referred to as "beer"), to sunlight or artificial light causes a significantly deleterious effect on the sensory qualities of the beverage by generating a so-called "skunky" flavor, which is sometimes also referred to as "sunstruck" or "light struck" flavor. It is known that the skunky flavor is the result of photochemical changes in the beverage that produce volatile sulfur-containing compounds. These sulfur compounds are thought to be formed at least in part by the reaction of other sulfur-containing compounds with photochemically degraded hop components in the beverage. Only very small amounts of these sulfur compounds are required to be present to impart the skunky flavor to the beverage and render it unacceptable.
The photochemical reaction is assisted by the presence of riboflavin, one of several photo-sensitizers in the beverage. The riboflavin emanates mainly from the malt, and to a minor extent via the hops, used in the production of beer and, according to common wisdom, action of yeast during the fermentation. (See Tamer et al. Enzyme Microb Technology 10:754-56, December 1988.) This photochemical reaction is a problem that to some degree has been the subject of a diverse remediation.
An approach that relies on primary packaging coloration either to exclude light or, at least, exclude those wavelengths of light that are particularly problematic, has been widely adopted. Such attempts to prevent beverages from becoming skunky involve enclosing the beer in cans or bottles made of protective, i.e., colored, glass, brown or amber being most efficient (see U.S. Pat. No. 2,452,968). These bottles reduce or eliminate the transmission to the beverage of light of wavelength shorter than about 560 nanometers. Such light is most harmful because it assists the riboflavin in enhancing the production of the undesirable volatile sulfur compounds.
Brown bottle glass has become a standard for the brewing industry for the purpose of avoiding the formation of skunky off-flavors, although in some circumstances green glass can be employed, generally with reduced efficacy.
Flint, or clear, glass--apart from the exclusion of the preponderance of ultraviolet wavelengths--is ineffective as packaging for traditional beer products that are susceptible to the formation of skunky off-flavors on exposure to visible wavelengths. In order to enjoy the visual aesthetic that is associated with this type of primary packaging (for example, the variously red, golden, or brown coloration and clarity of the beer beverage), the brewer is faced with the option of employing reduced hop extracts or taking the risk of the formation of skunky flavor. The skunky off-flavor, as previously stated, is, problematic. The use of reduced hop extracts, on the other hand, does not deliver the "noble" hop essences to the beverage that are associated with traditional beer products.
Another method developed to address the problem of "skunky" flavor production uses reduced isohumulones in place of hops or hop extracts. (See Verzele, M., et al., U. Inst. Brew. 73:255-57, 1967.)
Other methods involve adding light-stabilizing materials to the beverage. (See U.S. Pat. No. 4,389,421.) However, in some jurisdictions, the use of such compounds has not been approved. Further, many brewers are reluctant to use any additives at all but, rather, use hops or hop extracts in an effort to achieve traditional beer flavor.
Another alternative has been suggested by U.S. Pat. No. 4,389,421. This patent describes malt beverages that have added organic compounds possessing a 1,8-epoxy group and, optionally, another compound with a 1,4-epoxy group. The amount of the 1,8-epoxy compound is at least 0.25 ppb and, preferably, about one to six ppb by weight. Suitable sources of the 1,8-epoxy compounds are taught as including 1,8-cineole, or plant essences from cardamom, eucalyptus, peppermint, lavender, laurel, or star anise. A suitable 1,4-epoxy compound is 1,4-cineole. The addition of these compounds is taught as preventing the development of the "light struck" flavor in a range of malt beverages (for example, beer, ale, malt liquors, etc.).
The problem of skunky flavor has been the subject of research for many years, and such research continues. (See Sakuma et al., "Sunstruck Flavor Formation in Beer," American Society of Brewing Chemists, Inc., 162-65, 1991). This article also deals with the part believed to be played by riboflavin in the reaction that produces the "skunky" flavor and suggests that removing riboflavin from the finished beer may solve the problem. However, an acceptable means for achieving that suggestion has not been readily apparent, and the problem persists.
In accordance with currently accepted brewing science, the compound that is thought to be primarily responsible for this skunky off-flavor is 3-methyl-2-butene-1-thiol. The compound is believed to be formed when photochemical cleavage of side chains of hop-derived isohumulones is followed by the reaction of the resulting 3-methyl-2-butenyl radical with an undetermined sulfur-containing compound that is normally present in beer. Riboflavin, which is contributed from both vegetable and, to a much lesser extent, yeast sources, is generally accepted as being a photochemical sensitizer in this reaction sequence.